1. Field of the Invention
The present invention relates to semiconductor lasers, or stripe lasers, and particularly to a semiconductor laser diode structure for generating high frequency modulation of light intensity.
2. Description of the Prior Art
One standard technique for intensity modulating light at high frequencies is by directly modulating laser bias current, as disclosed in the published article of J.E. Bowers et al., "High-frequency constricted mesa lasers", Appl. Phys. Lett., V. 47, pp. 78-80 (15 July 1985). Another standard technique for intensity modulating light at high frequencies is by using an external electrooptic modulator, as disclosed in the published article of T. Sueta et al., "High Speed Guided-Wave Optical Modulators", J. Opt. Communications, V.3, pp. 52-58 (1982). However, these standard techniques are limited to frequencies of less than 30 gigahertz (GHz).
An alternative technique to the two above-described techniques involves the injection-locking of two discrete semiconductor lasers to the frequency modulation (FM) sidebands of a discrete master laser. Such an alternative technique will now be discussed by referring to the prior art system of FIG. 1 and the waveforms of FIG. 2.
The system of FIG. 1 relies on the principle that when the bias current (not shown) of a semiconductor master laser (ML) 11 is modulated sinusoidally at a modulating frequency f.sub.m, the emission or optical spectrum 12 of the master laser 11 splits up into several discrete lines or FM sidebands. These sidebands are separated by the modulation frequency f.sub.m and extend several tens of gigahertz on each side of the optical laser emission frequency or carrier frequency signal CF. Illustrated in FIG. 2 are exemplary optical sidebands N-1, N-2 and N-3 to the left of the carrier frequency signal CF and sidebands N+1, N+2 and N+3 to the right of the carrier frequency signal CF. More particularly, sideband N-3 is the third sideband left of (or below) the carrier frequency signal CF, while sideband N+3 is the third sideband on the right of (or above) the carrier frequency signal CF.
The FM modulated emission of the master laser 11 is transmitted to a beam splitter 13, which splits the emitted light into two beams. One beam passes through the beam splitter 13 to a semiconductor slave laser (SL) 15, while the other beam is reflected from the beam splitter 13 to another semiconductor slave laser 17. Thus, the FM modulated emission of the master laser 11 is injected or coupled by way of the beam splitter 13 into the semiconductor slave lasers 15 and 17. The emission frequencies of the slave lasers 15 and 17 are adjusted or tuned so that they approximately coincide with the exemplary sidebands N-3 and N+3, respectively, of master laser 11.
The frequency adjustment or tuning of the slave lasers 15 and 17 can be accomplished by changing the temperatures and/or bias currents of the slave lasers 15 and 17. After the frequency adjustments of the slave lasers 15 and 17 are carried out, light injected from the master laser 11 into each of the slave lasers 15 and 17 causes the slave lasers 15 and 17 to be injection-locked to the N-3 and N+3 sidebands of the master laser 11. These N-3 and N+3 sidebands are respectively designated in FIG. 2 as SL 15 and SL 17 frequencies.
Since light is injected from the master laser 11 into each of the slave lasers 15 and 17, the slave lasers 15 and 17 act as narrow band amplifiers for the particular sidebands that respectively coincide with their free-running frequencies. As a result, slave laser (SL) 15 amplifies the N-3 sideband and slave laser (SL) 17 amplifies the N+3 sideband of the master laser 11. The amplification bandwidth of each of the slave lasers 15 and 17 is narrow. Consequently, each of the slave lasers 15 and 17 only amplifies the particular sideband that its free-running frequency coincides with and is not affected by all of the other sidebands in the spectrum of the master laser 11 shown in FIG. 2.
This process of amplification is sometimes referred to as injection-locking. More specifically, injection-locking of slave laser 15 occurs when the optical emission of slave laser 15 takes on the same frequency and phase as the light from the injected sideband N-3 of master laser 11. In a similar manner, injection-locking of slave laser 17 occurs when the optical emission of slave laser 17 takes on the same frequency and phase as the light from the injected sideband N+3 of master laser 11. As a consequence of the slave lasers 15 and 17 being injection-locked by the respective sidebands N-3 and N+3 of the same master laser 11, the output light beams from those slave lasers will be phase coherent with each other.
The output light beams from the slave lasers 15 and 17 are then combined in a beamsplitter 19 to produce an optical beat frequency or intensity modulation frequency. To produce this optical beat frequency, the output beam from slave laser 15 is directly applied to the beamsplitter 19, while the output of slave laser 17 is sequentially reflected from mirrors 21 and 23 before being applied to the beamsplitter 19.
In response to the two beams from the slave lasers 15 and 17, the beamsplitter 19 produces a combined beam which contains an optical beat frequency which is equal to the optical frequency separation of the two slave lasers 15 and 17. This optical frequency separation is equal to the frequency separation of the sidebands N-3 and N+3 of the master laser 11.
It should be noted at this time that a fiber coupler (not shown) could be substituted for the beamsplitter 19 to produce the optical beat frequency between the two light beams from the slave lasers 15 and 17.
The combined beam of slave laser beams at the output of the beamsplitter 19 is photodetected by a photodiode 25 to develop microwave current signals. This photodiode 25 functions as a mixer to generate a difference frequency which is equal to the optical difference frequency between the frequencies of the two slave lasers 15 and 17.
A sum frequency is not generated at the output of the photodiode 25 because it is at a much higher frequency which is beyond the frequency response range of the photodiode 25. In addition, because of the phase coherence of the output light beams of the two slave lasers 15 and 17, which is a consequence of their being injection-locked by the sidebands of the master laser 11, the spectral width of the beat signal is very narrow.